The present invention relates generally to digital signal detector arrangements and in particular to a dynamic DC offset compensation arrangement in wireless systems.
Wireless technology provides multiple applications for voice and/or data transmission. Today's cell phone networks offer customers a plurality of services including digital data services, such as digital email, Internet access, etc. In future applications, such as third generation wireless networks, a plurality of new digital data services will be provided. In particular, Internet applications will be highly improved and made more practical, for example, via high speed digital data transmission. Other digital data application, not yet applicable in today's wireless transmission technology, will be adapted and implemented.
High speed wireless data applications require high data throughput at a significantly lower bit error rate than voice applications. Bit errors in voice applications are usually easy to recover or do not need to be fully recovered due to redundancy capabilities of the human ear; whereas, digital data applications often highly rely on the correctness of the submitted data. The quality of data transmissions in a digital environment highly depends on the quality of the transmission channel. Under severe channel conditions, the mobile device throughput is markedly affected due to retransmission of erroneous data packets, thus affecting the entire network throughput. This situation may be ameliorated by the use of antenna diversity and more sophisticated signal processing algorithms.
According to the prior art, decision feedback equalizers are used to compensate for the effects of the transmission channel, which can vary depending on the environment. A basic decision feedback equalizer (DFE) has a forward filter, a feedback filter, and a decision device. Decision feedback equalizers are effective against severe intersymbol-interference. Intersymbol-interference is an effect which creates distortion of the transmitted signal in a specific way. In a sequence of positive and negative symbol pulses, intersymbol-interference is the distortion of a symbol pulse within a particular symbol period caused by the smearing or spillover of symbol pulses of preceding and/or succeeding adjacent symbol pulses into the particular symbol period. The spillover of the preceding and/or succeeding symbol pulses will add to or subtract from the symbol pulse in the particular symbol interval, depending upon whether the adjacent interfering symbol pulses are positive or negative in value. In applications with mobile devices, intersymbol-interference occurs due to the multi-path profile of the mobile channel as well as the above mentioned smearing generated due to analog filtering. Unlike linear equalizers, decision feedback equalizers' decision errors propagate in the feedback branch, thus affecting the outcome of future bit decisions.
In digital communication receivers, an important problem is that of estimating the channel impulse response in the presence of a DC offset. In communication systems using frequency hopping, this needs to be done without knowledge about previous bursts. If the DC offset is constant throughout the burst, several techniques exist to remove the offset and subsequently perform the channel impulse response estimation (for example, by averaging the signal or least square circular fitting of the signal). Another option is the joint detection of DC and the channel impulse response, which seems to work particularly well for non-constant envelope signals. A special case is represented by the GSM 05.05 AM suppression test, which specifies an interfering source which starts transmission within the training sequence. The non-linearities in the front-end result in a DC step occurring inside the training sequence. In such a case, conventional DC estimation will fail and the subsequent channel impulse response estimation will lead to a corrupted channel impulse response, causing equalization to fail.
From the above, it is seen that it is desirable to provide compensation for a digital signal detector arrangement which considers DC offsets within a burst.